Advanced Perforation Modeling

ABSTRACT

A technique is provided for modeling flow simulations at downhole reservoir conditions and rock formations after performing wellbore perforations. By utilizing these flow simulations, a user may be able to simulate and compare different scenarios, thereby facilitating a more effective, profitable, and realistic choice of perforating systems and operating conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application claiming priorityto PCT Application No. PCT/US2014/028987 filed Mar. 14, 2014, whichclaims priority to U.S. provisional application No. 61/785,826 filedMar. 14, 2013 and U.S. provisional application No. 61/789,708 filed Mar.15, 2013, the technical disclosures of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to methods andapparatus for modeling flow simulations at downhole reservoir conditionsand rock formations after performing wellbore perforations.

2. Description of the Related Art

In the drilling of oil and gas wells, a wellbore is formed using a drillbit that is urged downwardly at a lower end of a drill string. Afterdrilling a predetermined depth, the drill string and bit are removed,and the wellbore is lined with one or more strings of casing or a stringof casing and one or more strings of liner. An annular area is thusformed between the string of casing/liner and the formation. A cementingoperation is then conducted in order to fill the annular area withcement. The combination of cement and casing/liner strengthens thewellbore and facilitates the isolation of certain areas of the formationbehind the casing to prevent the undesirable flow of hydrocarbonsbetween rock formations, which may, for example, contaminate aquifers,or to surface.

After a well has been drilled and completed, it is desirable to providea flow path for hydrocarbons from the surrounding formation into thenewly formed wellbore. To accomplish this, perforations are createdthrough the casing/liner string at one or more depth(s) which equate tothe anticipated depth(s) of hydrocarbon bearing strata. Predictivemodels are used to select an appropriate perforating system andperforating depth(s) for the wellbore.

SUMMARY

In one embodiment, a method of determining inflow of fluid enabled by aperforating system includes obtaining log data for a wellbore parameter;determining a rock penetration value using the wellbore parameter dataand a stressed rock test data; determining a clear tunnel value usingthe rock penetration value and the wellbore parameter data; anddetermining an inflow using the rock penetration value and the cleartunnel value.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic diagram of a perforating system of the presentinvention, prior to detonation of the perforating charges.

FIG. 2 is a schematic diagram of a perforating system of the presentinvention, after detonation of the perforating charges.

FIG. 3 illustrates example operations for a rock penetration model foraccurately modeling flow simulations at downhole reservoir conditionsand rock formations after performing wellbore perforations, according toan embodiment of the present invention.

FIG. 4 illustrates an exemplary wellbore having a perforation intervalbetween 4,100 m and 4,150 m.

FIG. 5 illustrates an exemplary porosity log over the perforationinterval of FIG. 4.

FIG. 6 illustrates the rock penetration value and clear tunnelmeasurement over the perforation interval of FIG. 4.

DETAILED DESCRIPTION

In order to produce hydrocarbon fluids from subterranean formations, aborehole is drilled from the surface down into the desired formations.Typically, cylindrical casing is placed and cemented into the borehole,thereby defining a hollow wellbore. In order for the hydrocarbon fluidsto flow from the surrounding formations into the wellbore and up to thesurface, it is necessary to perforate the casing. This is typically doneusing a perforating gun, a downhole tool that detonates explosivecharges at selected locations in order to form holes in the casing.

Because the fluids in the formation are under pressure, a choice must bemade whether to perforate the well with the bottom-hole pressure in thewellbore lower or higher than the formation pressure. The formercondition is referred to as “underbalanced” and the latter condition isreferred to as “overbalanced.”

FIG. 1 shows a schematic of a perforating system. A borehole 10 has beendrilled from the surface down through subterranean formations 12 thatcontain hydrocarbon formation fluids, namely oil and/or gas. A generallycylindrical casing 14 lines the wall of the borehole, defining thewellbore 16. A perforating gun 18 has been lowered into the well on awireline 20. The perforating gun includes at least one, and usuallyseveral explosive perforating charges 22. These charges are orientedsuch that when they are detonated, the force of the explosion will beprimarily directed outward toward the casing (e.g., horizontally outwardin FIG. 1). Detonation is triggered by a signal delivered through acontrol line from the surface (not shown in the figures).

When the explosive perforating charges are detonated, perforations 26are formed in the casing, as shown in FIG. 2. Explosive shaped chargeshave been used to perforate oil and gas wells. These small explosivedevices create metallic “jets” traveling at several km/sec, perforatingsteel casing, cement, and formation rock. Typically, multiple chargesare configured in a perforating “gun,” and initiated in rapid successionby a high explosive detonating cord, which has been initiated by adetonator.

The primary objective of perforating a cased wellbore is to establishefficient flow communication with the reservoir. The key perforatingparameters which influence reservoir deliverability include number ofshots per depth interval, angular arrangement of the shots relative tothe axis of the wellbore, depth of penetration (DoP) into the rockformation, perforation tunnel diameter, the nature of anypermeability-impaired (“crushed”) zone which remains surrounding theperforation tunnels, rock type, rock permeability, rock porosity,uniaxial compressive strength (UCS) of the rock, vertical stress actingon the rock as the result of overlying rock formations, reservoirpressure, reservoir temperature, fluid behavior under varying pressure,volume, and temperature (PVT) conditions, in situ gas-oil ratio (GOR)behavior, drainage radius of the well, and the radius of any damagedzone surrounding the wellbore as the result of drilling operations.

Shot density and phasing are fixed system parameters and, therefore,their values at downhole conditions are known. However, downhole valuesof perforation tunnel depth, diameter, and crushed zone characteristicscannot be known with certainty. These quantities must be estimated withpredictive models. By utilizing these predictive models, a user may beable to simulate and compare different scenarios and make an appropriatechoice for a perforating system and operating conditions. Therefore theaccuracy of these predictive models is an essential ingredient in theaccuracy of any well productivity or injectivity prediction.

Depth of penetration prediction of a downhole shaped charge (DoPprediction) has historically been largely determined by correlation toshaped charge penetration measured into unstressed concrete targetsunder ambient conditions at surface. This depth is then used to estimatean equivalent depth in the downhole rock. The actual reservoir strengthrelative to the downhole rock is then considered and a correction isapplied. Actual downhole stress (overburden and pore pressure) resultsin additional corrections. Additional effects such as water clearance,casing thickness, cement thickness, etc. are applied as additionalcorrections. However, reliance on unstressed concrete performance leadsto inaccurate results. For example, unstressed concrete performance maynot be indicative of downhole conditions in a wellbore for perforating.Shaped charges that are optimized to penetrate more effectively inunstressed concrete are typically not optimized for penetratingeffectively into stressed rock. Therefore, cement-based correlationsprove insufficient for flow simulations at downhole reservoir conditionsand (rock) formations. As a result, choice of perforating equipment forwells in specific regions may be based on limited simulations and priorsuccess (e.g., word of mouth) in that region.

In addition to being determined by unstressed concrete performance,downhole shaped charge DoP prediction has historically been determinedby reservoir properties that are averaged over a perforating interval.Examples of such averaged reservoir properties generally includepermeability, porosity, UCS, fluid PVT data, and in situ GOR behavior.Determining the downhole shaped charge DoP prediction based on averagedreservoir properties may also lead to inaccurate results, particularlydue to the fact that any of the averaged reservoir properties may varysignificantly over the perforating interval.

Certain embodiments of the present invention provide techniques foraccurately modeling flow simulations at downhole reservoir conditionsand rock formations after performing wellbore perforations. By utilizingthese accurate flow simulations, a user may be able to simulate andcompare different scenarios, thereby facilitating a more effective,profitable, and realistic choice of perforating systems and operatingconditions.

Rather than determining the downhole shaped charge DoP prediction basedon averaged reservoir properties over a perforating interval, multipleDoP predictions may be made over the perforating interval, such as on afoot by foot scale, and preferably at the greatest depth resolutionsupported by available data typically one-half to one foot intervalsbetween logging tool measurements. Determining the DoP prediction on afoot by foot scale may lead to more accurate flow simulation models(e.g., flow contributions across a perforation interval). In oneembodiment, DoP prediction on a foot by foot scale may be made usingwellbore logs that have been collected during the drilling of thewellbore (e.g., wireline logs, MWD logs, LWD logs, or mud logs). Thewellbore logs reflect property variations on a foot scale. A log-basedapproach captures variation of rock strength and permeability across theperforation interval and, therefore, provides a more accurate flowcontribution on a foot by foot scale. Examples of various wellbore logsthat may be used include gamma ray logs, neutron logs, density logs,sonic logs, and resistivity logs. FIG. 4 illustrates an exemplarywellbore having a perforation interval between 4,100 m and 4,150 m.

In another embodiment, seismic data that has been collected prior todrilling may be used for making the DoP predictions. Seismic data givesan estimate that is useful for rock strength estimation that can serveas a bound. 3D/4D seismic data may be useful for simulation ofre-perforations and for understanding declining production by reviewingpermeability changes. Lateral rock strength variation may be seen fromseismic analysis (e.g., after calibration). Examples of various seismicdata that may be used include data dealing with an estimated rockstrength, permeability changes (e.g., using time-lapse (“4D”) seismicdata), and lateral variation of rock properties for a particular depth.Seismic data may also be available for multi-well perforationsimulations. In another embodiment, seismic data may be used incombination with wellbore logs to refine DoP predictions.

Using the wellbore logs and/or the seismic data, and optionally alongwith drilling data (e.g., rate of penetration, drilling mud density, anddrilling mud properties), one or more of the following reservoirproperties may be calculated on a foot by foot scale: permeability,porosity, rock strength, vertical stress, reservoir pressure, reservoirtemperature, fluid PVT data, in situ GOR behavior, and damaged zoneradius. Determining the DoP prediction on a foot by foot scale based onthe above-mentioned parameters may lead to more accurate flow simulationmodels. Having the ability to compare flow simulations for differentperforating systems may assist in the choice of an appropriateperforating system for particular downhole reservoir conditions.

For certain embodiments, the wellbore logs described above may be usedalong with a rock penetration model for accurately modeling flowsimulations at downhole reservoir conditions and rock formations afterperforming wellbore perforations, as illustrated in FIG. 3. As describedabove, the logs, seismic data, or drilling data show foot by footvariation. FIG. 5 illustrates an exemplary porosity log over theperforation interval of FIG. 4. For a given perforation interval and aselected gun system, the actual rock penetration may be analyzed for aparticular depth, as will be described further herein. In oneembodiment, at step 302, for each depth within the perforation interval(e.g., 1 foot), the porosity at that depth may be obtained from aporosity log. At step 304, using stressed rock test data obtained from atesting facility, the rock penetration value maybe determined such as byinterpolation or extrapolation of experimental rock penetration databased on a given explosive weight and the porosity value at the depthwithin the perforation interval. FIG. 6 illustrates the rock penetrationvalue over the perforation interval of FIG. 4. In other words, a rockpenetration value may be determined based on the rock test data andactual reservoir conditions such as porosity obtained from a wellborelog at a particular depth, without depending on correlations fromunreliable test data gathered using unstressed concrete targets. Also,experimental tests in the flow labs have suggested rock-penetrationmodels are useful to simulate the effects and benefits of reactiveshaped charges. The clear tunnel of the penetrated formation is acontrolling parameter for flow prediction using Darcy's equation.

At step 306, a clear tunnel measurement of the rock penetration may bedetermined. Clear tunnel measurements reflect the actual tunnel that isopen for flow. Conventional cement penetration models assume that theentire perforated tunnel is open for flow, which is usually not thecase. Because the fluids in the formation are under pressure, the wellmay be perforated with the bottom-hole pressure in the wellbore lower orhigher than the formation pressure. The former condition is referred toas “underbalanced” and the latter condition is referred to as“overbalanced.” Using the selected underbalanced or overbalancedpressure condition, the determined rock penetration value describedabove, and porosity, the clear tunnel value may be determined such as byinterpolation or extrapolation of experimental clear tunnel measurementsobtained from stressed rock tests. FIG. 6 also illustrates the cleartunnel measurement over the perforation interval of FIG. 4.

As a result, an effective rock penetration value and clear tunnel valuemay be provided for a given gun system under selected reservoirconditions at a particular depth. At step 308, by using the determinedrock penetration value and clear tunnel value, the inflow value may becomputed for a particular perforating system. Having the ability tocompare flow simulations for different perforating systems may assist inthe choice of an appropriate perforating system for particular downholereservoir conditions.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on or encoded asone or more instructions or code on a computer-readable medium.Computer-readable media includes computer storage media. Storage mediamay be any available media that can be accessed by a computer. By way ofexample, and not limitation, such computer-readable media can compriseRAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic diskstorage or other magnetic storage devices, or any other medium that canbe used to carry or store desired program code in the form ofinstructions or data structures and that can be accessed by a computer.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of determining inflow of fluid of a perforating system,comprising: obtaining a log data for a wellbore parameter; determining arock penetration value using the wellbore parameter data and a stressedrock test data; determining a clear tunnel value using the rockpenetration value and the wellbore parameter data; and determining aninflow using the rock penetration value and the clear tunnel value. 2.The method of claim 1, wherein the wellbore parameter is porosity. 3.The method of claim 1, wherein determining a rock penetration valuecomprises interpolating or extrapolating the rock penetration valueusing the wellbore parameter date and the stressed rock test data. 4.The method of claim 1, wherein determining the clear tunnel valuefurther comprises using the bottom-hole pressure.
 5. The method of claim4, wherein the bottom-hole pressure is underbalanced pressure oroverbalanced pressure.